feat(ui): themed startup splash with deferred window show (#1030)

* feat(ui): add themed startup splash with deferred window show

Show a theme-aware loading splash before the Vite bundle mounts, hide the
native window until it paints, and use per-theme logo gradient colors.

* docs: note startup splash in CHANGELOG and credits for PR #1030

* docs: attribute PR #1030 startup splash to cucadmuh

src-tauri/tauri.conf.json

@@ -22,7 +22,8 @@
         "fullscreen": false,
         "decorations": true,
         "transparent": false,
-        "visible": true,
+        "visible": false,
+        "backgroundColor": "#1e1e2e",
         "dragDropEnabled": false
       }
     ],